Marine craft frequently require the capability for precisely controlled navigation in confined or restricted waters and, in particular, for precise control and maneuvering of a vessel at low speeds. A typical and frequently occurring example of such low speed, precisely controlled maneuvering is the docking of a vessel wherein the vessel must be brought into a precisely controlled position with respect to a docking area, at very low speed which is often at or below the minimum speed at which conventional propulsion and steering systems can provide the necessary control of the vessel.
Although conventional propulsion and steering systems have been and still are commonly employed in such low speed, precise maneuvering of a vessel, conventional rudder and propeller systems present a number of difficulties in such maneuvers and typically require that a vessel be piloted by an experienced operator familiar with the particular and often unique characteristics of the vessel with respect to the steering and propulsion responses of the vessel and the responses of the vessel to such factors as, for example, wind and currents. In vessels having, for example, a conventional single propeller and rudder system or an odd number of propellers, the central propeller will typically generate an unbalanced transverse thrust that will tend to turn the vessel toward the port or the starboard side of the vessel, depending upon whether the central propeller has a right or left hand blade pitch and whether the propeller is rotating in a clockwise or counterclockwise direction. This effect may be mitigated or avoided in vessels having an even number of propellers by arranging the propellers with opposing blade pitches so that the propellers rotate in opposite directions, but still may occur if the engine speeds are different, thereby resulting in an unbalanced lateral thrust. This effect is accentuated at low speeds, and the problem is compounded because of the interactions between the angle and direction of water flow over the rudder or rudders caused by the propeller or the propellers and by the motion of the vessel through the water. While experienced pilots familiar with the characteristics of a given vessel or vessels may employ these effects while maneuvering a vessel, such experience is often lacking, and can result in undesirable outcomes, even for smaller vessels ranging from scratches, dents and damage to a docking area to major damage to or even the sinking of a vessel.
The above described problems with conventional propeller and rudder systems has resulted in the development of lateral thrusters mounted at or in the bow or bow and stern of a vessel and using transversely mounted propellers to generate lateral forces on the bow and/or stern of a vessel, thereby facilitating turning of the vessel and allowing a vessel to be moved or positioned laterally, including allowing a vessel to be held stationary against winds and currents. In general, bow thrusters are mounted in transverse tunnels extending from one side to the other side of the vessel at or near the bow, which is generally narrow compared to the mid-section of a vessel. Stern thrusters, however, because of the differing shapes assumed by the sterns of various vessels, may for example be mounted internally in the hull with inlet and outlet ports, in transverse passages or tunnels in a fin-like region of the keel forward of the propellers and rudders, or in cylindrical ducts or housings mounted transversely on the stern or transom of the vessel. In other implementations, thrusters may be mounted in or on retractable housings that are stored within the hull along the keel, when not in use, and that are extended below the keel when required.
Examples of conventional tunnel thruster installations of the prior art are shown in FIGS. 1A through 1C in which FIG. 1A illustrates a tunnel thruster system 1 that includes a tunnel thruster propulsion mechanism 10 having a single reversible propeller 12 mounted in a transverse tunnel 14 extending transversely to the keel axis 16K of the hull 16 of the vessel 18. A tunnel thruster system 1 is typically installed as far forward as possible in the hull to maximize the leverage effect around the pivot point and as deep as possible below the waterline to avoid any air from being sucked from above the water surface into the tunnel 14, which would significantly decrease the effectiveness of the tunnel thruster. As illustrated, the tunnel thruster propulsion mechanism 10 includes a drive unit 10A, typically an electric or hydraulic motor or a connection to an internal combustion engine, a motor mount 10B supporting a transmission and propeller assembly 10C, and a gearing or flexible drive shaft(s) for converting the rotation of a drive shaft 10D, connected with the drive unit 10A, into rotation of a propeller drive shaft 10E which drives a propeller 12. FIG. 1B illustrates a tunnel thruster system 1 similar to that of FIG. 1A, but in which the tunnel thruster propulsion mechanism 10 includes two opposed propellers 12A, 12B wherein the pitches of the blades of propellers 12A, 12B and the gearing of propeller assembly 100 are arranged so that the propellers 12A, 12B operate cooperatively to generate lateral thrust. In this regard, the pitches and drive trains of the propellers 12A and 12B may be arranged so that the propellers 12A and 12B either rotate in the same rotational direction or are counter-rotating, that is, the propellers 12A and 12B rotate in opposite rotational directions. FIG. 1C illustrates the central portion 14A of a transverse tunnel 14 of a tunnel thruster system 1 having opposed reversible propellers 12A, 12B, similar to that illustrated in FIG. 1B.
As discussed above, stern mounted tunnel thruster systems 1 are generally similar to the bow mounted tunnel thruster systems 1, illustrated in FIGS. 1A-1C, but may be mounted to a vessel differently due to the different shape of the stern regions of a vessel, as compared to the bow regions of the vessel. As described above, stern tunnel thruster systems 1 may, for example, be mounted internally in the hull with inlet and outlet ports, in transverse passages or tunnels in a fin-like region of the keel forward of the propellers and the rudders, or in cylindrical ducts or housings mounted transversely on the stern or transom of the vessel. In other implementations, the tunnel thruster systems may be mounted on or in retractable mountings, stored within the hull along the keel when not in use, and extended below the keel when required, and may be rotatable about a vertical axis to allow the thrust, generated by the thruster system, to be directed at a range of angles relative to the keel of the vessel or possibly mounted internally within inlet or outlet ports.
Tunnel thruster systems, however, suffer from a number of disadvantages and limitations that are inherent in the flow of water through a cylindrical passage, that is, the tunnel of a tunnel thruster system, and the interaction between a propeller and the water flowing in the tunnel. For example, the thruster tunnel inherently restricts the volume of the water flowing through the propellers region of influence, thereby correspondingly restricting the thrust than can be generated by the propeller, and the interaction between the water and the tunnel boundaries presents a significantly higher flow resistance compared to a propeller acting in an open flow region, both of which result in significantly reduced efficiency compared to a propeller acting in an open flow region. The effects of the tunnel on water flow characteristics also often result in the generation of high levels of noise due to propeller cavitation, as discussed in further detail below.                Considering the inherent disadvantages and limitations of tunnel thruster systems in further detail, and considering bow thruster systems as exemplary of all forms of tunnel thrusters, FIG. 2A is a diagrammatic illustration of the flow of water into and through a tunnel 14 of a conventional tunnel thruster system 1 of the prior art and illustrates the effects of the shape of the transition region 20 between the entrance of tunnel 14 and the hull 16 and, in particular, the effects of a too sharp or badly rounded tunnel 14 to hull 16 configuration. As indicated therein, a too sharp or badly rounded hull 16 to tunnel 14 flow transition region 20, such as at flow discontinuity 20D, or any other form of discontinuity or too abrupt a change in the path of fluid flow, such as a discontinuity or too sharp a gradient in the wall 22 surface, will result in the formation of a urbulence region 24T near the wall 22 surface wherein a turbulence region 24T is characterized by macroscopic turbulence, a detached boundary layer, eddies and vortices while the flow of water in a non-turbulent inner zone 24L is characterized by an undetached boundary layer and little or no turbulence, eddies or vortices. The turbulence, eddies and vortices in a turbulence region 24T results in and determines the magnitude of a reduction in the rate of flow of water in the turbulence region 24T, that is, a reduction in the mean axial flow speed of the water near the tunnel wall 22. This, in turn, results in a slowing of the fluid flow adjacent the walls 22 of the tunnel 14 and may adversely affect the effectiveness and the efficiency of the thruster propeller 12, 12A or 12B. That is, and as illustrated in FIG. 2C, the reduction in the mean axial speed (VA-speed of advance) of the water flow near the wall 22 will, in turn, result in and determine an increase in the angle between the velocity of the water relative to the blade and pitch line, that is, the angle of attack of the blade of the propeller 12, 12A or 12B. In addition, the turbulence in the water flow around the propellers creates irregular and unpredictable velocity variations along the propeller blade surfaces, thereby making it difficult to optimize the propeller design in order to reduce noise and increase efficiency and performance.        
The solution to such fluid flow problems in thruster tunnels 14 that have been most commonly recommended and adopted in the prior art, as is illustrated in FIG. 2B, is to round the juncture between the hull 16 and the tunnel wall 22 to thruster tunnel flow transition region 20 so as to avoid flow separation and the formation of turbulence regions 24T, with the most common recommendation being that the optimum radius of the transition region 20 be on the order of 10% of the tunnel 14 diameter. This solution is believed to not only reduce the water flow characteristics leading to the turbulence region 24T, but to allow the thruster 10 to draw water from a region around the tunnel 14 opening in the hull 16. The increased movement of water over a larger area results in a suction pressure acting on the hull surface that increases the effect and efficiency of the thruster 10 proportional to the increase in area of the hull surface on which the suction pressure is exerted due to the increased radius of the transition region 20.
It is well known, however, that the achievement of the recommended optimum hull 16 to the thruster tunnel flow transition region 20 shape presents significant design problems in, for example, achieving the necessary hull structural strength, significant increases material and hull space costs and requirements and construction time and effort, so that these solutions of the prior art generally have proven unsatisfactory.
The present invention provides a solution to these and related problems of the prior.